Vent cleaning of rubber molds

ABSTRACT

Vents and micro-structures of rubber molds may become clogged with rubber that is difficult to remove. These vents and micro-structures can be cleaned of rubber, even if heat aged, by subjecting the mold to high temperatures in the presence of a solvent to devulcanize any rubber present. If the rubber used with the mold being cleaned includes carbon black, a solvent may be used to dissolve the devulcanized polymer, leaving the carbon black which can be removed by water jets or other cleaning means.

This application is a U.S. National Stage entry under 35 U.S.C. § 371 ofInternational Application No. PCT/CA2018/050925, filed Jul. 30, 2018,which claims the benefit of U.S. Provisional Application No. 62/538,897,filed on Jul. 31, 2017. The entire contents of each of theseapplications is incorporated herein by reference.

TECHNICAL FIELD

The current disclosure relates cleaning structures, such as molds, usedin forming rubber products having fillers, such as carbon black, and inparticular to removing vulcanized rubber remaining in the structuresafter forming the rubber products.

BACKGROUND

Molds used to manufacture rubber products like car tires often havesmall holes to help with the manufacturing process. The holes allow gasto vent during the vulcanization process in making the rubber productsuch as tires. While rubber is being injected or pressed into the mold,there are risks of air bubbles forming between the rubber and the mold,and if not allowed to escape, the air bubbles can prevent the rubberfrom getting in all the intricate patterns of the mold, thus producing amalformed product. These vent holes can sometimes be seen on new tiresas small hair-like structures, known as vent spews. The holes of themolds can be very small, such as fractions of a millimeter.

During use, the vents can become filled with vulcanized rubber, whichcan be difficult to remove. As the hole can be many millimeters in depththey can be difficult to clean by conventional methods. Current cleaningtechnologies include lasers, CO₂ dry ice blasting, and abrasive particleblasting. They are primarily for cleaning the surface of molds and arenot that practical for cleaning the small holes.

SUMMARY

In accordance with the present disclosure, there is provided a method ofremoving vulcanized rubber having filler from a structure comprising:subjecting the structure and the vulcanized rubber to a high temperatureof between 250° C. and 320° C. in the presence of a devulcanizationsolvent for a reaction time sufficient to devulcanize the vulcanizedrubber; subjecting the structure and devulcanized rubber to a dissolvingsolvent to dissolve rubber polymer from the devulcanized rubber; andremoving filler remaining from the vulcanized rubber from the structure.

In a further embodiment, the method further comprises pre-treating thestructure and the vulcanized rubber with the devulcanization solvent.

In a further embodiment of the method, the pre-treating is carried outat an elevated temperature at about the boiling point of thedevulcanization solvent.

In a further embodiment of the method, the vulcanized rubber on thestructure comprises heat aged vulcanized rubber.

In a further embodiment of the method, the structure comprises one ormore mold surfaces of a mold.

In a further embodiment of the method, at least one of the one or moremold surfaces comprises at least one of: a surface within a vent of themold; a surface of a micro-slot vent of the mold; a surface of amicro-structure of the mold; a surface of a spring vent within the mold;and a surface between puzzle mold pieces.

In a further embodiment of the method, the devulcanization solvent andthe dissolving solvent are the same solvent.

In a further embodiment of the method, the devulcanization solvent andthe dissolving solvent comprise toluene.

In a further embodiment of the method, the devulcanization solventcomprises water and the dissolving solvent comprises toluene.

In a further embodiment of the method, the devulcanization solvent andthe dissolving solvent are selected from the group consisting of:toluene; benzene; xylene; cyclohexanes; hexanes; heptanes; andchloroform.

In a further embodiment of the method, removing the filler comprisesremoving the filler using one or more of: a water jet; an air jet; and apressure washer.

In a further embodiment of the method, the filler comprises carbonblack.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects and advantages of the present disclosure will becomebetter understood with regard to the following description andaccompanying drawings in which:

FIG. 1 depicts a Horikx plot;

FIG. 2 depicts a method of cleaning vulcanized rubber from a mold;

FIG. 3 depicts heat aged vulcanized rubber particles;

FIG. 4 depicts the heat aged vulcanized rubber particles of FIG. 3 afterthe devulcanization process of the current disclosure; and

FIG. 5 depicts the remaining rubber particles of FIG. 4 after rollingbetween fingers.

DETAILED DESCRIPTION

Molds are used to make a wide variety of products from rubber. Therubber vulcanization process typically exposes the rubber that is placedinto the mold to temperatures in the range 130 to 170° C. for severalminutes before the product is ejected from the mold. Any rubber thatremains on surfaces of the mold and is not removed will be exposed tomultiple heating cycles. As is known, when rubber is exposed to elevatedtemperatures for extended times it heat ages and can significantlychange its chemical composition/structure and physical properties.Although not wishing to be bound by theory, it is believed that heataging vulcanized rubber changes the number and proportion ofmonosulfilic, disulfilic, and polysulfilic crosslinks in the rubberconverting polysulfilic to disulfilic and monosulfilic crosslinks. Thisresults in considerably more cross links and a change in the physicalproperties of the heat-aged vulcanized rubber. Exposure to air atelevated temperatures can also cause some oxidation of the materialremaining on mold surfaces. The heat aged rubber becomes harder and morebrittle, with an increase in modulus and a decrease in tensile andelongation at break.

When the mold needs to be cleaned, any vulcanized rubber remaining onmold surfaces may or may not be physically or chemically identical tothe rubber products being manufactured depending upon whether theremaining rubber has been heat aged. The cleaning process describedfurther herein is able to remove both vulcanized rubber as well asheat-aged vulcanized rubber from mold surfaces. While the processdescribed herein is able to remove remaining rubber from large surfacesof the mold that are easily accessible, it is particularly well suitedfor removing remaining rubber from surfaces that have limited physicalaccess such as surfaces of vents in the mold. Removing normal or heataged vulcanized rubber from within these vents is not simple and quitedifferent from removing it from the surface of a mold where directaccess is possible.

The molds used in forming vulcanized rubber products may include smallvent holes to allow gas to escape from the mold. The molds may haveother small structures similar to the vent structures such as cavities,slots, micro-slots, micro-grooves or other small structures. Thestructures of the molds can be cut directly into the mold, for exampleby a laser, or may be formed by sandwiching mold pieces together havingthe appropriate spacing between them, which may be referred to as puzzlemolds. Further, the molds may include separate structures that areinserted into the mold such as spring vents. Spring vents that have aspring valve as well as micro-slots and micro-cavities within the vent.Rubber can be trapped within, or on, any of the surfaces of thesestructures of the mold and can be difficult to remove as a result of thelimited physical access to the surfaces due to the size and shape of thestructures. The rubber remaining on these surfaces can be come heataged, making its removal more difficult, and ultimately leading to amold that is no longer useful.

The remaining of the description refers to a mold cleaning process forremoving vulcanized rubber, including heat-aged vulcanized rubber, fromthe surface of mold vents. The same cleaning process can be used toremove vulcanized rubber, including heat-aged vulcanized rubber, fromall surfaces of the mold, including surfaces that have limited physicalaccess due, at least in part, to the size and shape of mold structures.That is, while the mold cleaning process is described with reference tocleaning vents for simplicity and clarity of the description, it can beused to clean surfaces of the mold structures including, for examplecavities, slots, micro-slots, micro-grooves, spring vents or other smallstructures formed in or on the mold, or inserted into the mold.

The mold cleaning process described herein includes the devulcanizationof the rubber in the vents of the mold, even if the rubber has beenheat-aged without affecting the mold itself, such as by corrosion.Further, the mold cleaning process does not cause the formation of“char”, which bonds carbon black particles together making it verydifficult to remove them from the vent. Once the rubber has beendevulcanized the polymer is removed from the vent by solvent extraction,and the remaining carbon black, or other types of filler, that remainsin the vent can be removed.

FIG. 1 depicts a Horikx plot. Removing the devulcanized polymer bysolvent extraction is a complex problem and can be explained using a“Horikx Plot”. A Horikx Plot shows what happens when you devulcanizerubber. It looks at the percentage of the polymer that can be removed bya solvent versus the percent devulcanization, or the decrease in crosslink density, for the two processes that occur during devulcanization,that is breaking the sulphur-sulphur cross links formed by thevulcanization process, or breaking the polymer molecules or chainsthemselves. Generally, when devulcanizing rubber that will be reused, itis desirable to break the sulphur-sulphur cross links without breakingthe polymer molecules. For this, the process should be carried out underconditions that produces a product with properties in the region of thetriangle 102 on the curve of the Horikx plot 100. For mold cleaning, itis not desirable to be in a region where only a small amount of thepolymer can be removed by a solvent, instead it is desirable to be inthe region of the curve represented by the square 104. In this region,some chain scission can be expected to occur, however for mold cleaningthis is not a large problem since the devulcanized products are notbeing reused and mixed into new rubber. While chain scission isacceptable for mold cleaning, it is desirable to be minimized, or atleast kept low, since small molecules resulting from the chain scissioncan form a bond between adjacent carbon black particles causing “char”that would make the remaining carbon black residue hard and difficult toremove from the vents.

As breaking the polymer chains can result in char formation that canbond the carbon black particles together, it is preferable that thepolymer be removed as fast and soon as possible during thedevulcanization process when the polymer molecules are still long and donot form a char. In order to remove polymer quickly, a solvent that willdevulcanize the rubber and, at the same time, remove it from the ventsmay be used. Toluene can be used as a solvent for both thedevulcanization and dissolving steps as it devulcanizes the rubber andthen dissolves the polymer into the toluene being used in one step.Other solvents may include, for example, benzene, xylene, cyclohexanes,hexanes, heptanes, and chloroform.

It is possible that different solvents could be used for thedevulcanization and dissolving steps, for example using a first solventto devulcanize the rubber in the first step followed by an exposure to asecond solvent that extracts the polymer as a second step. It ispossible to use, for example, water as the devulcanizing solvent andtoluene as the dissolving solvent.

When the polymer in rubbers are heated above a certain temperature thevery long polymer molecules break down into smaller and smallermolecules, referred to as depolymerization, eventually forming a “char”that binds the remaining carbon black particles together. Removing thepolymer molecules before they completely break down and form a char isdesirable when cleaning a mold, as well as using temperatures thatreduce an amount of depolymerization that occurs. Thermogravimetricanalysis has shown that breaking down the polymer molecules to onessmall enough to become volatilized (i.e. pyrolysis) takes temperaturesin excess of 350° C.

The devulcanization process for the heat aged rubber trapped in themold's vents will be different to the devulcanization process for normalrubbers as the heat-aged rubbers are harder and have significantly moremonosulfidic sulphur cross links that need to be broken affecting theprocess.

The devulcanization process may be affected by the width and length ofthe vents. However, an understanding of the process conditions can beachieved by looking at particles of the rubber that is being used in themolds that has been heat aged. In testing of the process describedherein vents as narrow as 30 microns have been cleaned. Conditions thatallow the devulcanized polymer to be extracted from these vents aredescribed further below. The polymer molecules that are removed duringthis devulcanization process can be recovered from the solvent andstudied. The dissolved polymer extracted from the vents can be recoveredby evaporating the solvent to dryness. While some depolymerizationoccurs, the molecular weights of the extracted polymer are still veryhigh at about 100,000.

The rubber in the vent is primarily vulcanized rubber/polymer and carbonblack. The cleaning is a two-step process. In the first step the rubberin the vent is completely devulcanized in a solvent at elevatedtemperatures and pressures, and, either after the devulcanization or atthe same time as the devulcanization, the devulcanized rubber moleculesare removed with the solvent before charring can occur, that is thebreakdown of the polymer to small molecules that can bind with thecarbon black forming a hard lumps of carbon. In the second step thecarbon black particles that remain are removed from the vent. Since thepolymer has been extracted from the carbon black particles during thefirst step, the cleaning of the carbon black from the vents issignificantly easier and can be achieved with simple cleaning techniquessuch as with a pressure washer or water jet. Other means of removing theremaining carbon black from the vents may include air jets or otherremoval methods as will be apparent to one of ordinary skill in the art.

The solvent used in the devulcanization process should not be corrosiveto the molds being treated. For that reason, organic solvents arepreferable. The solvent used may dissolve the polymer/rubber moleculesbeing processed so that the polymer may be extracted as the rubber isdevulcanized. Different solvents are available for different rubberswhich have varying degrees of solubility. These include a range ofaromatic and aliphatic compounds, including for example cyclohexanes,hexanes, heptanes, benzene, xylene, chloroform, THF, etc. Based on testresults toluene is preferable although other solvents can be used.Further, although described as using a single solvent fordevulcanization and extraction of the polymer, it is possible to usedifferent solvents for the devulcanization of the rubber and for theextraction of the devulcanized rubber polymer.

The solvent should also completely devulcanize the heat aged vulcanizedrubber so that all of the vulcanized rubber remaining in the molds canbe cleaned. Toluene has also been found to be very effective.

Solvents such as toluene are sometimes referred to as swelling solventsand can be used to swell rubber, which reduces it physical propertiessuch as strength etc. In such a process, rubber is brought in contactwith the swelling solvent. Rubber can typically take up 2 to 5 times itsown weight in swelling solvent. Once swollen, shear forces are typicallyapplied to the rubber to break it up. This is a type of devulcanizationprocess that is usually referred to as a reclamation process. However,such reclamation processes do not work in cleaning molds. The vulcanizedrubber in the vents cannot swell as it is contained by the walls of thevents, nor can shear forces be applied to it. The cleaning processdescribed here is quite different from the reclamation of rubber using aswelling solvent as no swelling can take place with the rubber that iscontained by the walls of the vents.

In addition, the geometry of the vents makes it very difficult for thesolvent to enter into the rubber trapped within the vent. The rubbertrapped in vent is much like a very thin film of rubber, however, theonly place the solvent can enter into the rubber are the edges, whichare, for example, 30 microns wide. The area of these edges exposed tothe solvent is very small and so diffusion of solvent into the rubber isvery slow, especially at room temperature, and can take a very longtime. The devulcanization process for the mold cleaning is based onsolvent entering the rubber and breaking the sulphur bonds. Gettingsolvent into the rubber is therefore an important consideration. Onemight think that as the temperature rises and the pressure in thereactor increases the solvent is forced into the rubber, however thisappears to be incorrect. Tests have shown that pressure can impede theentry of solvent into the rubber. For example, if the rubber (fromtires) is placed in water (at room temperature), and pressure is appliedimmediately, the rate uptake of water is about a quarter of that withoutpressure being applied. Tests conducted have shown that, under normalcircumstances, the high pressures appears to compact the rubber makingit much more difficult for solvent to enter into the rubber and thepolymer molecules to be extracted after devulcanization.

Rubber molecules can normally move relative one another as the rubberis, for example, stretched. The rubber may be modeled as a tube. In theTube Model, the rubber molecules are folded up like a concertina withintubes and the rubber molecules can move freely within these tubes. Thereis therefore a void space between the rubber molecules, both between thetubes and inside the tubes, which under normal circumstances is filledwith air. This can be thought of as the space through which solventsenter into the rubber. The compaction effect resulting from highpressure means the void space between the rubber molecules becomessmaller and experiments have shown that it then becomes much moredifficult for the solvent to enter into the void space and the uptake ofsolvent is significantly reduced. The compaction effect thereforereduces the uptake of solvent by the rubber and inhibits thedevulcanization process.

Research also appears to show that this compaction effect also causesadditional entanglement of the polymer molecules, and, as a result, thedevulcanized material becomes much harder and the polymer more difficultto remove. It has also been found during this research that replacingthe air with solvent minimizes this effect. That is replacing the air,which is compressible, with a liquid that is not compressible, preventsthe physical entanglement and makes it much easier to remove thedevulcanized rubber molecules.

Pre-treatment of the rubber with solvent to fill this void space withsolvent, and in particular a liquid versus a compressible gas, preventsor reduces this compaction of the rubber as pressure is applied to it.In the process described here pre-conditioning is advisable to preventthis compaction effect that limits the entry of the solvent into therubber and the removal of the polymer molecules after devulcanization.

However, as noted above, the replacement of the air by solvent is veryslow at room temperature. Heating the solvent increases the rate atwhich it can be replaced but if the temperature becomes too high thepressures created by the solvent, its saturated vapour pressure, cancause the rubber molecules to be compacted. Temperatures slightly abovethe boiling point at atmospheric pressure of the solvent are preferred.Replacing the void space with solvent, and in particular with a solventat an elevated temperature of approximately the boiling point of thesolvent, is referred to as pre-conditioning of the rubber. Whilepre-treatment is not essential, it can significantly reduce processingtimes in the reactor.

Processing times for cleaning vulcanized rubber, including heat agedvulcanized rubber, from molds can range from 1 to 6 hours depending onhow much solvent is introduced during the pre-conditioning stage as wellas how much polymer needs to be removed from the rubber and the ventgeometry of the mold. Even when the rubber is devulcanized the polymerneeds time to diffuse out of the devulcanized rubber, and out of thevent. The more distance the polymer needs to diffuse out of the vent thelonger it takes. The processing time may further depend upon the rubbercharacteristics as well as the degree of the mold contamination.

The process conditions, and in particular the temperature and time, usedto devulcanize the rubber will depend on the rubbers being used in themold, as well as the level of mold contamination and the vent geometry.That is, different types of rubber need to be processed under slightlydifferent conditions. The devulcanization reaction kinetics will beslower at lower temperatures, and so require longer reaction times, andfaster at higher temperatures, and so require shorter reaction times.Devulcanization can occur are typically in the 250-320° C. range withpressures at or above the saturated vapour pressure of the solvent atthe process temperature. Although reaction time will vary, typicalreaction times may be in the range of 1-6 hours.

Reactors to accomplish this process are described in U.S. Pat. Nos.9,403,136 and 9,278,324. The reactors may be joined together to allowfor energy recovery by reducing the pressure in one reactor at the endof the process, causing the solvent to evaporate, and then condensing itin a second reactor. Venting the reactor at the end of the process, i.e.when at high temperature and pressure, in spurts can be used to helpbreak up any carbon black particles in the vents as liquid solventtrapped within the particles evaporates. Standard pressure washers canbe used to remove the remaining carbon black or other fillers from thevents. Other washing techniques may be used to remove remaining carbonblack and other fillers.

FIG. 2 depicts a method of cleaning vulcanized rubber from a mold. Themethod 200 can be used to clean mold surfaces, including inaccessiblesurfaces such as the interior surface of vents, of vulcanized rubber,and heat-aged vulcanized rubber, that remains on the surface afterremoving the vulcanized rubber product from the mold. The mold,including remaining volcanized rubber that may or may not be heat aged,is subjected to high temperatures in the presence of a solvent (202) inorder to completely devulcanize the vulcanized rubber. The mold, andrubber in the mold, may first be pre-treated or pre-conditioned withsolvent at an elevated temperature to introduce solvent into the rubberto prevent or reduce compaction of the rubber in the mold at elevatedpressures. The high temperatures are maintained for a reaction period oftime in order to allow all of the vulcanized rubber to be devulcanized.The pressure of the reaction may be at or above the vapour pressure ofthe solvent. The devulcanized rubber polymer is dissolved from the moldinto a solvent (204), which may be the same solvent used indevulcanizing the rubber or a different solvent. Once the polymer isextracted, only carbon black will remain on the mold, and it is removedfrom the mold (206). The carbon black can be removed from the mold bywashing or other processes including, for example using a water jet, orpossibly an air jet.

In a first test, particles of heat aged tire rubber were devulcanized intoluene at 300° C. for one hour to look at the devulcanized sample andits ability to be removed by a pressure washer. While as noted above,rubber particles behave quite differently to the rubber in vents, thistest demonstrates that devulcanization and the removal of the polymerfrom devulcanized rubber is possible. FIG. 3 shows 1-2 mm particles ofheat aged vulcanized rubber before they were processed. FIG. 4 showswhat remains of the particles of FIG. 3 after processing. As can be seenfrom FIG. 4 some of the particles were completely dispersed into thesolvent, while some did not and retained their original shape. As can beseen in FIG. 5 when the remaining particles of FIG. 4 were rolledbetween a tester's fingers, the particles completely broke down intofine powder. This demonstrates that these processing conditions areideal for this heat aged rubber, and the carbon black that remains afterdevulcanization, can be easily removed.

In a second test, cleaning of vents in a mold segment that were full ofrubber was achieved. A mold segment with micro-vents 30 microns widethat had been used in a tire manufacturing process and was completelyblocked with hard vulcanized rubber was pre-conditioned and processed intoluene at 290° C. for two hours. The vents were then cleaned with awater jet/pressure washer. The vents were then observed with a lightbehind the slot and was shown to be completely clean. In a similar testwith a vent with a diameter 400 microns were cleaned the same way.

In a third test a mold segment with spring vents blocked with rubber wascleaned. The rubber was trapped between the surfaces of the valve in thespring vent and had also been pushed all the way through and out theback of the valve. The mold segment was cleaned by processing thesegment in toluene, with pre-conditioning at 285° C. for two hours. Thevalves, which were closed, opened on treatment. The residual carbonblack was then flushed out of the spring vents with solvent.

The mold cleaning process can remove or clean out vulcanized rubber thathas been trapped in all types of molds for manufacturing rubber productsand all types of venting systems used in them. The removal can be fromthe surface or from cavities or vents in the molds, includingmicro-vents. The process uses a solvent that can completely devulcanizenormal or heat aged rubber without forming a char. The mold cleaningprocess uses a solvent that devulcanizes the rubber and removes thedevulcanized rubber polymer before char is formed. A solvent could beused to devulcanize the rubber and then a second solvent used to extractthe polymer however, this may be less desirable than using a singlesolvent for both the devulcanization and the extraction steps. Althoughtire rubber was tested, the mold cleaning process can be applied to alarge number of different rubber types as long as a solvent is availableto devulcanize the rubber that does not corrode the molds. Depending onthe rubbers used and solvents used the process temperatures may be inthe range 250-320° C. with pressures at or above the saturated vapourpressure of the solvent at the particular processing temperature. Themold cleaning process is applicable to all rubbers used to manufacturetires as well as for cleaning vulcanized rubber from other molds.

What is claimed is:
 1. A method of removing vulcanized rubber havingfiller from a structure used during forming a vulcanized rubber product,the method comprising: subjecting the structure and the vulcanizedrubber on at least one surface of the structure used during forming thevulcanized rubber product to a high temperature of between 250° C. and320° C. in the presence of a devulcanization solvent for a reaction timesufficient to devulcanize the vulcanized rubber; subjecting thestructure and devulcanized rubber to a dissolving solvent to dissolverubber polymer from the devulcanized rubber leaving the filler on the atleast one surface of the structure; extracting the dissolved rubberpolymer from the structure; and removing the filler remaining on the atleast one surface of the structure from the structure.
 2. The method ofclaim 1, further comprising pre-treating the structure and thevulcanized rubber with the devulcanization solvent.
 3. The method ofclaim 2, wherein the pre-treating is carried out at an elevatedtemperature at about the boiling point of the devulcanization solvent.4. The method of claim 1, wherein the vulcanized rubber on the structurecomprises heat aged vulcanized rubber.
 5. The method of claim 1, whereinthe structure comprises a mold.
 6. The method of claim 5, wherein atleast one surface of the mold comprises at least one of: a surfacewithin a vent of the mold; a surface of a micro-slot vent of the mold; asurface of a micro-structure of the mold; a surface of a spring ventwithin the mold; and a surface between puzzle mold pieces.
 7. The methodof claim 1, wherein the devulcanization solvent and the dissolvingsolvent are the same solvent.
 8. The method of claim 7, wherein thedevulcanization solvent and the dissolving solvent comprise toluene. 9.The method of claim 1, wherein devulcanization is carried out at anelevated pressure and the devulcanization solvent comprises water, andwherein the dissolving solvent comprises toluene.
 10. The method ofclaim 1, wherein the devulcanization solvent and the dissolving solventare selected from the group consisting of: toluene; benzene; xylene;cyclohexanes; hexanes; heptanes; and chloroform.
 11. The method of claim1, wherein removing the filler comprises removing the filler using oneor more of: a water jet; an air jet; and a pressure washer.
 12. Themethod of claim 1, wherein the filler comprises carbon black.